feat: add private relay for end-to-end-encrypted remote access (#2087)

Adds OpenChamber Relay — an opt-in way to reach an instance from a phone,
browser, or another desktop from anywhere, with no open inbound ports, no
tunnel, and no shared LAN. The instance dials outbound to a relay; all app
traffic (HTTP, the event stream, terminal, dictation) is multiplexed and
encrypted through a single connection per client, so the relay only ever
forwards opaque ciphertext.

Transport
- End-to-end-encrypted channel over WebCrypto (ECDH P-256 -> HKDF ->
  AES-256-GCM) with a capability-negotiated handshake and a small
  HTTP/SSE/WebSocket multiplexing protocol. A byte-compatible JS host mirror
  is cross-checked by tests.
- Host: outbound connection manager, per-client tunnel dispatcher to the local
  server over loopback, reuse of the existing instance identity key, and
  management routes. Disabled by default; explicit opt-in.
- Client: plugs into the existing runtime layer (runtime-fetch/-url/-switch/
  -auth, event pipeline, terminal, dictation) so features work over the relay
  unchanged; direct-URL and Electron realtime-proxy paths are untouched.

Pairing & UX
- Relay section in Settings -> Remote Instances (live status, QR/link pairing,
  revocation via the existing client-token list) and the mobile connect flow.
- Frame batching and idle-gated keepalive keep tunnel message volume low
  without affecting streaming smoothness.

Security
- The tunnel is transport only; the server authenticates every tunneled
  request exactly as for a direct remote client.
  fragments only. The relay stores no keys, tokens, or payloads.

Operability
- The endpoint can be pinned to a self-hosted rel
  paired clients inherit it from the offer automatically.
- Relay module DOCUMENTATION.md and a relay-trans
  invariants that future WebSocket/streaming changes must follow.

The relay transport is complete and tested; the UI for enabling and pairing
is gated behind openchamber_relay_gate and stays

AGENTS.md

@@ -115,6 +115,12 @@ Server-side text-to-speech services and summarization helpers for `/api/tts/*` e
 
 - Module docs: `packages/web/server/lib/tts/DOCUMENTATION.md`
 
+##### relay
+
+Host side of the private relay: outbound E2EE tunnel that lets remote clients reach this instance through OpenChamber-hosted relay infrastructure without inbound exposure. Load the `relay-transport` skill before changing it or any WebSocket/streaming endpoint that rides it.
+
+- Module docs: `packages/web/server/lib/relay/DOCUMENTATION.md`
+
 ##### tunnels
 
 Tunnel provider setup and runtime helpers for exposing OpenChamber over remote URLs.
@@ -341,6 +347,7 @@ Project skills live under `.agents/skills/*/SKILL.md`. Before editing, agents **
 | Settings pages, settings dialogs, configuration UI, or visual/layout changes inside Settings | `skill({ name: "settings-ui-patterns" })` |
 | Drag-to-reorder, sortable lists/chips/grids, or `@dnd-kit` behavior including touch/mobile and wrapping variable-width items | `skill({ name: "drag-to-reorder" })` |
 | iOS Simulator preview/control for the mobile app, `serve-sim`, simulator taps/typing/gestures/rotation, or headless install/launch workflows outside Xcode | `skill({ name: "serve-sim" })` |
+| WebSocket/SSE/streaming endpoints (terminal, dictation/voice, event stream, notifications), opening a WebSocket in shared UI, runtime transport refactors (`runtime-fetch`/`runtime-url`/`runtime-switch`/`runtime-auth`), the private relay tunnel, or anything under `packages/ui/src/lib/relay` or `packages/web/server/lib/relay` | `skill({ name: "relay-transport" })` |
 
 Skill docs are the source of truth for detailed patterns. Do not duplicate their full guidance here; load the skill and follow it before making matching changes.